DE2854002A1 - HEAT RESISTANT NICKEL STEEL ALLOY - Google Patents

Description

The invention relates to a nickel-steel alloy which is particularly suitable in the hardened state as a heat-resistant material with a low coefficient of expansion.

From "Materials in Design Engineering", Nov. 1965, hardenable nickel-steel alloys with a low coefficient of thermal expansion of, for example, 7.2 to 9.9 times 10 [high] -6 per ° C. and a high inflection point of, for example, 371 to 482 ° C. are known . However, these alloys are notch sensitive at 538 to 649 ° C. Furthermore, it is known from US Pat. No. 3,705,827 to counteract this by means of a heat treatment which prevents recrystallization, while US Pat. No. 4,006,011 proposes alloy engineering measures for this purpose. In practice, however, there are higher requirements, especially with regard to the workability; for example, very different forging temperatures, solderability and good machining properties, in addition to a low coefficient of expansion and a high turning temperature, are desirable.

The invention is based on the object of a nickel-steel- To create an alloy that also withstands stress concentrations that occur at high temperatures. The solution to this problem is a nickel-steel alloy with 34 to 55.3% nickel, up to 25.2% cobalt, 1 to 2% titanium, a total amount of niobium and half the tantalum content of 1.5 to 5.5%, up to 2% manganese, up to 1% chromium, up to 0.03% boron and less than 0.20% aluminum, the remainder including iron-related impurities. In the hardened state, such an alloy has a turning temperature of at least 343 ° C, a coefficient of thermal expansion of at most 9.9 times 10 [high] -6 per ° C up to the turning temperature and a room temperature yield strength of at least 773.6 N / mm [high ] 2; their iron content is 20 to 55%.

The alloy advantageously contains at least 10% cobalt, in particular with a total nickel and cobalt content of 51 to 53%, in order to set a high turning temperature.

The accompanying elements including deoxidation and refining agent residues include up to 0.01% calcium, 0.01% magnesium, 0.1% zirconium, 0.5% silicon and up to 1% each of copper, molybdenum and tungsten. Sulfur and phosphorus are harmful; their contents should therefore not exceed 0.015% in each case.

The tantalum content should not exceed 10% of the niobium content. The resulting difference between the niobium and tantalum contents is irrelevant. For the sake of simplicity, the alloys in question then contain 1.5 to 5.5% niobium or niobium and tantalum. On the other hand, however, the alloy can also contain up to 11% tantalum.

The alloy is hardened preferably for at least 8 or 16 hours at 732 to 593 ° C and can be annealed beforehand. In order to ensure particularly favorable values for the expansion coefficient, the turning temperature and the yield point in the hardened state, the alloy should meet the following conditions:

A: (% Ni) +0.84 (% Co) -1.7 (% Ti +% Al) +0.42 (% Mn +% Cr) less than or equal to 51.5

B: (% Ni) +1.1 (% Co) -1.0 (% Ti) -1.8 (% Mn +% Cr) -0.33 (% Nb + 1/2% Ta) greater than or equal to 44 , 4

C: (% Nb + 1/2% Ta) (% Ti) -0.33 (% Cr) greater than or equal to 2.7

and contain 21 to 51.2% iron, 11% tantalum, or at least 26.5% iron and 5.5% niobium without tantalum.

A particularly favorable combination of the coefficient of thermal expansion and the yield point results when the alloy contains 35 to 39% nickel, 12 to 16% cobalt, 1.2 to 1.8% titanium and a total content of niobium and half the tantalum content of 3.7 to 4.8%, in each case up to 1% manganese and chromium, up to 0.012% boron, preferably 0.003 to 0.012% boron, and up to 0.1% aluminum, the remainder including impurities caused by the smelting iron.

The equation values A are preferably at most 47.5 and B at least 48.8 and C at least 4.8. Such alloys have a coefficient of thermal expansion of at most 8.1 times 10 [high] -6 per ° C, a turning temperature of at least

416 ° C and a room temperature proof stress of at least 914.2 N / mm [high] 2. Melting such an alloy is easier if the alloy contains a total nickel and cobalt content of 51 to 53%, about 1.5% titanium and about 0.3% manganese and chromium.

The coefficient of thermal expansion can be calculated according to the formula

0.248 (% Ni) +0.209 (% Co) -0.427 (% Al +% Ti) +0.104 (% Mn +% Cr) -7.39

calculate while turning the turning temperature according to the equation

26.9 (% Ni) +29.6 (% Co) -57.2 (% Al) -28.2 (% Ti) -47.0 (% Mn +% Cr) -8.90 (% Nb + 1 / 2% Ta) -509

calculated. The values in question can easily be converted from Fahrenheit to Celsius. The coefficient of thermal expansion is an average value for the range from room temperature to the turning temperature. The equations themselves were mathematically and statistically created on the basis of the data from numerous dilatometer tests with alloys falling under the invention, but also with alloys just outside the invention.

The steel alloy in question is free of stress cracks even at high temperatures and is particularly suitable as a material for turbine parts.

In order to demonstrate the low sensitivity to stress cracking, samples were exposed to air for long periods at elevated temperatures, for example in the context of notch fracture and stress cracking tests at 538 ° C and 649 ° C. Stress corrosion cracking tests were also carried out under load, in which a hoop sample made of tape was held in an oven. The macroscopic examination showed the formation of cracks and crack propagation in the area of grain boundary oxidation in alloys that are sensitive to stress cracking.

The proposed nickel-steel alloy can be melted in the usual way. In this respect, melting in an induction furnace, in air or in a vacuum, or vacuum-arc melting or remelting are possible. The steel alloy has good deformability and can accordingly be hot-cold and cold deformed. In the case of a recrystallizing annealing following the hot deformation, in particular a high notch strength results. Hot-cold forming is understood here to mean forming at temperatures around 16 to 166 ° C. below the recrystallization temperature. In the recrystallized state, the alloy has a structure with an equiaxed grain and accordingly isotropic properties. The good cold-hot deformability of the alloy is of great economic importance because it allows forging, rolling or other deformation up to temperatures below the recrystallization temperature and accordingly makes intermediate annealing or heating superfluous.

The alloy can be deformed at temperatures in the range from 1149 ° C. to below the recrystallization temperature; it can be annealed for recrystallization for one hour to fifteen minutes at 927 to 1038 ° C., depending on the material thickness and deformation stresses from the deformation below the recrystallization temperature, following hot-cold deformation. The alloy is preferably annealed for one hour at 927 ° C. or fifteen minutes at 1038 ° C. or correspondingly long at intermediate temperatures in order to set a fine-grained billet structure or a somewhat coarser-grained strip structure. A fine-grain structure ensures low crack sensitivity or good notch breaking strength as well as high room temperature strength. Regardless of this, some alloys have a low sensitivity to stress cracking, both with a coarse and with a fine-grain structure.

In this context, fine-grained is a recrystallization structure with a grain size of up to ASTM 5, normally ASTM 5 to 8, while a coarse-grained structure means a grain size of ASTM 4.5 or larger, normally ASTM 2 to 4.

The recrystallizing annealing aims at a homogeneous solid solution at least the majority of the small gamma formers. In any case, the purpose of the annealing is not to bring the carbon into solution. The annealing is preferably followed by water quenching in order to maintain the solid solution until the next treatment stage, although slower cooling rates, for example air cooling, are also possible.

The alloy is hardened for at least eight hours at 621 to 732 ° C. The hot-worked and possibly hot-cold-worked or cold-worked alloy is preferably solution-annealed before hardening, although this is not essential. Curing for eight hours at 718 ° C. with an oven cooling rate of 55 ° C./h to 621 ° C. and holding for eight hours at this temperature with subsequent air or oven cooling to room temperature is particularly favorable. Other temperatures from 732 to 843 ° C. are also suitable for improving the fracture toughness and / or the above-mentioned grain boundary cracking resistance.

In general, the alloy with both a fine and a coarse-grain structure in the hardened state has a yield strength of at least 773.5 N / mm [high] 2 and a tensile elongation at room temperature of at least 10%.

The invention is explained in more detail below on the basis of exemplary embodiments.

A steel alloy 1 with 36% nickel, 17% cobalt, 3% niobium and 1.5% titanium was melted in the vacuum induction furnace. The alloy was alloyed with boron and calcium and then cast in a vacuum to form a block. The composition of the alloy can be seen from Table I below. The ingot was then hot rolled to a thickness of 0.635 mm and then cold rolled into 0.152 mm thick sheet.

Samples measuring 1.905 x 10.16 mm and 0.95 x 10.16 mm were cut from the sheet metal and solution-annealed for fifteen minutes at 1038 ° C., quenched in water, and annealed for eight hours at 718 ° C. at a rate of 55 ° C / h cooled from the annealing temperature to 621 ° C in the furnace and then kept at this temperature for eight hours and cooled in air. The samples had a recrystallized structure with a grain size of ASTM 4 to 5. The 0.2 proof stress at room temperature and the tensile strength, elongation and necking at 538 ° C. were determined on transverse samples using the results shown in Table II; At room temperature, these show the good mechanical properties of the steel alloy with a yield strength of 896.6 N / mm [high] 2 and an elongation of 14.

In order to test the sensitivity to stress cracking, further hardened transverse specimens were ground to a fineness of 320. The sheet metal thickness was then measured, the required sample length according to ASTM standard G 39-72 for the preparation of ironing samples for the stress corrosion cracking test with compensation for the elongation of the clamping device was calculated and the samples were then cut to size. The sample ends were ground by grinding with a kind of chisel edge for point contact with the sample holder. When clamping in the specimen holder, the clamping bolts were tightened in such a way that a load of 1055 N / mm [high] 2 resulted at the test temperature of 538 ° C. The sample holder with the sample was kept at a temperature of 538 ° C. in a hood furnace. The furnace had a peephole for observing the sample, for example at intervals of 4 to 24 hours. In the event of a breakage after 294 hours, the tool life was 294 hours.

The aforementioned service life proves the very good stress cracking resistance of the cold-rolled transverse specimen. Although a quarter hour anneal at 1038 ° C is favorable with respect to isotropic

Properties, transverse specimens provide a better criterion for stress cracking resistance than longitudinal specimens.

Table I contains further test alloys 2 to 11 falling under the invention as well as comparative alloys A to I. The total content of niobium and tantalum given in table I includes a tantalum content of up to 2%, based on the total content; however, alloy 1 contained 0.02% tantalum, alloy H 0.03% tantalum and alloys 7, A, B and I less than 0.01% tantalum. Accidental accompanying elements were found to be below 0.2% silicon, 0.15% copper, 0.05% molybdenum, 0.015% sulfur and 0.010% phosphorus.

Table II shows values for A, B and C as well as the corresponding dilatometrically determined expansion coefficients (CTE) and turning temperatures (WT). Similarly, Table III contains mechanical property data for alloys within and outside of the invention. In connection with the stress cracking resistance, the time of the last observations before the break and the time of the first observation after the break are given. The service life is accordingly between the two time values. At the end of the observation period, samples that are still free of cracks are marked with o.R. marked. The samples for the creep tests had the dimensions 0.51 x 3.18 mm, but for alloys 7 to 9 they were 0.64 x 3.18 mm.

Samples were worked out from square forged billets with an edge length of 1.43 mm of alloys 4 to 6 and C to F and examined for their notch strength at 649 ° C and room temperature and for their strength in the short tensile test at a temperature of 649 ° C. The test results are shown in Table IV. The billets were gradually reduced in cross section by 0.64 mm at 1121 ° C with intermediate heating up to an edge 1.75 mm long, hammer-forged, cooled to 871 ° C in the forging machine and then forged down to an edge length of 1.43 mm and cooled in air. After the heat treatments given in Table IV, the grain size was ASTM 7 to 9. The heat treatment A corresponds to a one-hour solution heat treatment at 885 ° C. with water quenching, followed by an eight-hour heat treatment at 718 ° C. with furnace cooling at 55 ° C./h and an eight-hour heat treatment 621 ° C with air cooling; In contrast to this, the temperature of the solution treatment in heat treatment B is 927 ° C.

The data in Table IV show the beneficial effect of an aluminum content of at most 0.2% with regard to an optimal combination of strength, toughness and breaking strength at stress concentrations, for example in the case of a notched specimen.

In connection, the data in Tables III and IV show that the breaking strength improves with long-term exposure if the maximum aluminum content is not exceeded. Alloy 2, for example, had an excellent service life in view of its low aluminum and chromium contents. This generally applies to alloys with low aluminum contents of, for example, 0.05% and, at the same time, low chromium contents of, for example, 0.3 to 0.5%. In contrast, the tool life was extremely short with an aluminum content of 0.006% and a chromium content of 0.58%.

Table IV shows the load at the time of breakage. Initially, the samples were subjected to a load of 492 N / mm [high] 2 for 48 hours, which was increased by 35 N / mm [high] 2 at intervals of 8 to 16 hours. The samples of the room temperature tests had a diameter of 0.64 mm and a length of 3.18 mm, while the sample length of the hot tests with the same diameter was 2.54 mm. The core samples had a diameter of 0.45 mm and a straight length of 1.82 mm with a notch factor K [deep] t of 3.6.

Welding tests according to the Varestraint process demonstrated the low sensitivity of the proposed steel alloy to weld cracks in comparison to the Eiselstein-Bell-Nickel-Cobalt steel alloy G with a low coefficient of expansion. Alloy G was produced in the usual way in a vacuum induction furnace. The data of the welding tests with an amperage of 190 A, a voltage of 13.1 V and a feed rate of 12.7 cm / min result from the following table V.

Electron beam welding of alloys 10 and 11 in the as-rolled condition and in the hardened condition after annealing for 15 minutes at 1038 ° C showed the same good weldability as conventional comparison alloys, although the welding behavior of the two samples was different despite the same grain size from ASTM 5. The few weld samples already demonstrated the superior resistance of sample 10 to under-seam cracks. In addition, the metallographic examination of alloy 10 after the bending test did not reveal any cracks either in the as-rolled condition or in the heat-treated condition.

The proposed steel alloy can be hot-rolled, hot-cold-rolled and cold-rolled or forged without difficulty and has excellent machinability. In addition, the alloy can also be excellently soldered as a wrought alloy, for example as sheet metal or strip, regardless of the respective material combination. In addition, the alloy has high strength and ductility both in the cold-hot and in the cold-deformed state at temperatures of up to 1038 ° C, for example.

The proposed steel alloy is suitable as a material for objects which, like turbine parts, are subjected to a cyclical temperature change at room temperature, 316 ° C, 538 ° C or 649 ° C, for example for seals, supports, flanges, shafts, bolts and housings of gas turbines .

Table I.

Table I (continued)

Table II

Table III

Table IV

Table IV (continued)

Table V

Claims (14)

1. Age-hardening nickel-steel alloy with 34 to 55.3% nickel, up to 25.2% cobalt, 1 to 2% titanium, a total content of niobium and half the tantalum content of 1.5 to 5.5%, up to 2% manganese , up to 1% chromium, up to 0.03% boron and less than 0.20% aluminum, the remainder including iron-related impurities. 2. Alloy according to claim 1, but containing at least 10% cobalt. 3. Alloy according to claim 1 or 2, which, however, contains 35 to 39% nickel, 12 to 16% cobalt, 1.2 to 1.8% titanium, a total content of niobium and half the tantalum content of 3.7 to 4.8% , in each case up to 1% manganese and chromium, up to 0.012% boron and up to 0.1% aluminum, the remainder including impurities caused by the smelting iron. 4. Alloy according to claim 3, but containing at most 0.05% aluminum. 5. Alloy according to one or more of claims 1 to 4, which, however, meet the conditions A: (% Ni) +0.84 (% Co) -1.7 (% Ti +% Al) +0.42 (% Mn +% Cr) less than or equal to 51.5 B: (% Ni) +1.1 (% Co) -1.0 (% Ti) -1.8 (% Mn +% Cr) -0.33 (% Nb + 1/2% Ta) greater than or equal to 44 , 4 C: (% Nb + 1/2% Ta) (% Ti) -0.33 (% Cr) greater than or equal to 2.7 enough. 6. Alloy according to claim 5, which, however, satisfies the condition A less than or equal to 47.5, B greater than or equal to 48.8 and C greater than or equal to 4.8. 7. Alloy according to one or more of claims 1 to 6, the total nickel and cobalt content of which, however, is 51 to 53%. 8. The alloy of claim 7 but containing 1.5% titanium and a total of 0.3% manganese and chromium. 9. Alloy according to one or more of claims 1 to 8, which, however, contains at least 1.5% niobium including up to 10% tantalum. 10. The alloy according to claim 9, but containing 3.7 to 4.8% niobium including at most 10% tantalum. 11. Use of a hardenable alloy according to claims 1 to 10 as a material for objects which, like turbine parts, must have high heat resistance, a low coefficient of thermal expansion and a high turning temperature. 12. Use of a hardenable alloy according to claims 1 to 10 as a material for objects that have a coefficient of thermal expansion of at most 9.9 times 10 [high] -6 per ° C and / or a turning temperature of at least 343 ° C and / or at room temperature must have a 0.2 yield strength of at least 773.6 N / mm [high] 2. 13. Use of a hardenable alloy according to claims 1 to 10 as a material for objects that have a coefficient of thermal expansion of 8.1 times 10 [high] -6 per ° C and / or a turning temperature of at least 416 ° C and / or a 0, 2 proof stress of 914.2 N / mm [high] 2 at room temperature. 14. Use of an alloy according to claims 1 to 11 after fifteen minutes to one hour of optional annealing 927 to 1038 ° C and curing for at least eight hours at 621 to 732 ° C.